Precipitated calcium carbonate is used widely as a mineral pigment or filler in the production of commodities such as paper, toothpaste, paint and plastics and high grade PCC is used in formulations of pharmaceuticals. PCC is produced through a reaction process that utilizes calcium oxide (lime), carbon dioxide and water. This precipitation reaction is capable of producing three distinct polymorphs (calcite, aragonite and vaterite) depending on the exact reaction conditions used. The main commercial types of PCC are acicular aragonite or calcite with the following morphologies: rhombohedral, prismatic, colloidal, acicular and scalenohedral and for each shape it is possible to adjust the aggregation level, particle size, size distribution and surface area by changing the process parameters.
Most commercial PCC carbonation reactions are operated in reactors with a low energy intensity of the mixing (less than 10 kW/m3). Therefore, if a homogeneous solution should be ascertained during carbonation, and thereby also a homogeneous product, a relatively low viscosity of the slaked lime is required (Brookfield viscosity less that 1000, spindle 3 100 rpm). Therefore, most commercial PCC is produced in the carbonation step at a final dry between 12 and 24% solids Because of the low solids further downstream processing (concentration) is needed if the fillers are to be transported over longer distances. Concentration of fillers in industrial scale will normally involve high shear or high pressures giving rise to breakage of some of the PCC particles or PCC aggregates. Breakage of PCC particles frees Ca(OH)2 thereby increasing the pH of the PCC solution. Since commercial PCC solutions are often sold at a pH between 8 and 10 it is often necessary to add a pH stabilising step.
For PCC produced in a conventional low energy intensity reactor, it is possible to increase the solids to reach a dry matter content of approximately 35% dry matter by mechanical dewatering of the carbonated product, such as by centrifugation, decanting or filtration. This will, however, decrease the steepness of the particle size distribution as shown by an increase in the steepness number calculated as the mass amount of the 75% particle size fractile divided with the mass amount of the 25% mass fractile. Analyses performed by the inventors on full scale PCC production show that the 75%/25% mass fractile ratio may increase from 1.5 to 2.0. Furthermore a secondary pH stabilising step is avoided.
Several processes for PCC precipitation have been described in the art in which PCC with high dry matter content is produced in processes involving the use of unslaked lime: U.S. Pat. No. 6,761,869, U.S. Pat. No. 6,602,484, U.S. Pat. No. 6,699,318, U.S. Pat. No. 6,475,459 and WO 03/106344 all provide processes involving carbonation of unslaked lime or processes in which the steps of slaking and carbonation are combined. In each case, however, the process requires high energy intensity, high pressure and/or the use of large gas volumes which makes the processes unsuitable for manufacturing PCC in large scale and at a reasonable cost. Furthermore, these processes cannot be used in the manufacture of all the commercially important PCC morphologies mentioned above.
Other specialised processes for the manufacture of particular PCC morphologies are available: U.S. Pat. No. 5,695,733 provides a process for converting aggregated scalenohedral PCC to aggregated rhombohedral PCC by controlling conductivity during the reaction and U.S. Pat. No. 6,022,517 describes a process for the manufacture of acicular calcite or aragonite calcium carbonate having clusters of rod shaped or needle-shaped acicular crystals by carbonation of calcium hydroxide or calcium oxide in the presence of water-soluble aluminium compounds. While these processes may involve carbonation of unslaked lime, they do not address the need for an efficient and economical process suitable for production of all commercially important PCC morphologies.
Finally, when unslaked lime is used in the processes reviewed above, it is suggested to add the unslaked lime directly into the carbonation reactor. In a conventional low energy reactor, however, addition of unslaked lime directly into the reactor is problematic as there is a relatively large flow of gas through the reactor. The gas is saturated with water and since lime is hygroscopic the gas flow results in settlement of lime on the various equipment parts leading to dysfunction of mechanical parties in the reactor and to the formation of non-uniform products. Furthermore, lime added directly to a carbonation reactor might give rise to large calcium concentrations at the addition point. This might cause seeding and thereby a broader particle size of the final product.
Hence, an improved process for the manufacture of uniform precipitated calcium carbonate in a low energy intensity reactor and at high solids would be advantageous for practical and economic reasons. In particular, it would be desirable to devise a principle for increasing the solids in carbonation reactions that can be used in the preparation of all commercially relevant PCC morphologies.